Digital photography and imaging provide cost-effective alternatives for capturing images, but known methods of producing durable, hardcopy prints of digitally printed areas are at least as expensive as traditional photographic methods. Further, with increasing use of various printing and imaging technologies in the publishing industry as well as in the home, protecting imaged or printed documents against abrasion, water alcohol, other liquid spills, ink smear, fading, blocking or other image-degradation processes and effects has become an important consideration. Such protection is particularly desirable for printed or imaged documents produced with water-based (water-soluble) or other liquid inks, as well as documents printed or imaged with toner. These are commonly used in ink-jet printing, offset printing, electrophotography and the like.
Photography provides an easy and reliable way to permanently capture images for a variety of uses. While photographs provide durable images, they are prone to scratches, have poor resistance to light and ultraviolet radiation (which causes photographic images to fade over time), and degrade when exposed to water, other liquids or to vapors of such liquids. Traditional photography uses harsh and expensive chemicals, requires silver recovery, and involves a process requiring several intermediate steps of handling negatives. While photographic processes can be automated, such automatic processing machines are expensive and bulky and do not eliminate the inherent problems of chemical exposure and handling negatives. Additionally, producing large prints (larger than the traditional 3-by-5 inch or 4-by-6 inch prints) can be quite expensive.
Hot and cold laminates are the most common methods used to protect printed areas. However, laminates tend to be expensive, typically costing 6 to 80 cents per square foot for materials. The labor-intensive nature of producing durable prints via lamination also increases the cost of such prints. Laminates may be applied on one or both surfaces of the print. One-sided lamination may lead to excessive curling of the final print, whereas two-sided application can be very expensive in terms of material and labor costs and may excessively increase the thickness of the final print. Adhesives used for cold laminates may be tacky at room temperature, leaving a sticky residue at the edges of the prints. Additionally, binders used in creating cold laminates are typically water-based, which means the print may delaminate if exposed to excessive water or other liquid. Laminates are also susceptible to trapped air pockets, which are viewed as image defects. Most importantly, care must be taken to ensure that the laminates are accurately aligned to the media, and such alignment is especially critical for a continuous web laminate. These are just some of the deficiencies of traditional laminates.
Liquid overcoats are also commonly used to protect photographic prints and are becoming more popular as protective coatings for inkjet printed areas. Typical systems for applying these overcoats rely on roller coating or gravure type systems to dispense, gauge, and apply the coating. Smaller systems typically apply the overcoat off-line, rather than being an integral part of a single printing and coating unit. Larger systems used by the printing industry are in-line, but require extensive monitoring. Both systems require significant manual cleaning or intervention to maintain the components that contact the liquid.
Liquid overcoats tend to be slightly less expensive than laminates (6–18 cents per square foot). However, because currently available systems must be cleaned frequently and regularly monitored, these methods of using liquid overcoats are just as labor-intensive as the lamination methods, if not more labor-intensive.
Additionally, many of the overcoat formulations have residual odors before and/or after application, and some people find these odors offensive or even harmful.
Ultraviolet (UV) light curable liquid overcoats are also available, such as the overcoats commonly used to protect magazine covers. In such a UV-curable system, the liquid is first applied to the surface of the print and then cured to yield a solid, durable, protective coating. Because these liquids are widely used in large volumes for the magazine industry, their cost tends to be significantly lower than most other overcoat options. However, the systems used to apply such UV-curable overcoats tend to be more complicated and costly than other liquid overcoat systems, due to the multi-step application and cure process. Additionally, many of the overcoat formulations have strong odors, some of which are harmful or offensive to people. Furthermore, there are potential safety problems associated with the handling of the potentially hazardous liquids used in this process.
Malhotra (U.S. Pat. No. 5,612,777 assigned to Xerox), Tutt & Tunney (U.S. Pat. No. 5,847,738 assigned to Eastman Kodak Co.) and Tyagi et al. (U.S. Pat. No. 5,783,348 assigned to Eastman Kodak Co.) disclose methods of applying a clear, scratch-resistant, lightfast, toner coating onto printed areas. Malhotra describes photocopied color images created by first, depositing color toner on a charge retentive surface; second, depositing a clear polymer toner material onto the charge retentive surface; and third, transferring and fusing the color toner and clear polymer toner material onto a substrate. Tutt & Tunney describe a process of depositing and fusing a clear polymer toner on inkjet printed areas. Tyagi et al. describes a similar process for coating clear toner over silver halide printed areas.
Similar electrostatic coating methods are also commonly used in the commercial painting industry to powder coat products, parts, or assemblies. One powder coating method charges a powdered paint using an air gun outfitted with an electrode before spraying the charged paint onto an electrically grounded object. Alternatively, an electrically grounded object may be immersed in a charged, fluidized bed of paint particles (typically referred to as“fluidized bed powder coating”).
Another Malhotra patent (U.S. Pat. No. 5,906,905 assigned to Xerox) discloses a method of creating photographic quality prints using imaging such as xerography or ink jet by, first, reverse reading toner printed areas on a transparent substrate and then adhering the transparent substrate to a coated backing sheet, coated with a polymeric lightfastness material.
The application of thermal film material on a thermally printed substrate is also disclosed. Nagashima (U.S. Pat. No. 4,738,555 assigned to Toshiba) discloses the use of a thermal printhead to thermally transfer a transparent protective layer of wax, vinyl chloride, vinyl acetate, acrylic resin, styrene or epoxy onto the thermally printed medium substrate.
Tang et al. (U.S. Pat. No. 5,555,011 assigned to Eastman Kodak) discloses a means to ensure that a thermal film that is being applied to a thermally printed surface has a clean break at the edge of the transfer. It describes a thermal film transfer method having a transport system which moves a dye-donor web and a receiver medium (i) in a forward direction along their respective paths past a thermal head, so that heat from the thermal head causes an area of the thermal film material coating between leading and trailing edges to transfer from the dye-donor web to the receiver medium and (ii) in a reverse direction along their respective paths such that the area of the thermal film material which is transferred to the receiver medium breaks cleanly at the trailing edge from a non-transferred area of the thermal film material that remains on the dye-donor web as the web support separates from the medium.
Abe et al. (U.S. Pat. No. 5,954,906 assigned to Canon) discloses a method for protecting and covering a printed material on a substrate with a pressure-sensitive protective covering material with at least (a) a first flexible substrate, (b) an adhesive layer, (c) a solid resin layer, and (d) a second flexible substrate, stacked in this order.
The packaging, printing, and decorating industry uses colored ribbons, known as thermal transfer foils, hot stamping foils, roll foils, and transfer printing foils, for marking or decorating. This market uses solid fill colored ribbons or uniquely patterned ribbons to emboss lettering, patterns, barcodes, or insignias on wood, paper, leather, plastic, fabric, or metal parts. Examples include holograms on credit cards, metalized insignias on baseball cards, corporate logos on business cards, or colored or metalized designs on greeting cards. The hot stamp foiling process involves the transfer of the coatings from a carrier ribbon onto a substrate via a combination of heat and pressure.